Epoxy resins contain a number of the reactive oxirane ring structures commonly called “epoxy.” The most commonly used resins are derivatives of bisphenol A and epichlorohydrin shown in structure I below. However, other types of resins (for example bisphenol F type) are also common to achieve various properties.
Epoxy coatings are formed by the reaction of a poly(epoxide)-based oligomer or resin with a polyfunctional active hydrogen compound hardener or curing agent. This curing reaction crosslinks the epoxy resin polymer and solidifies it into a durable coating. The focus of this invention is two component or 2K systems, with separate epoxy and polyamine hardeners (a polyamine pre-reacted with some epoxy or dimer fatty acid curing agent).
Organic solvents have been used to manage viscosity and maintain compatibility between the epoxy resin and hardener components, but are VOCs. Water use is more environmentally friendly, but requires surfactants since epoxy resins are hydrophobic and water reactive and therefore incompatible with water.
Epoxy resins are also available in various molecular weights to provide unique properties to the final coating. Epoxy molecular weights of about 300 Daltons are generally liquid at room temperature; those of 500 molecular weight are semi-solid, while those of 700 and above are solid in the absence of solvent. Molecular weights much higher than those listed are also used. Epoxy resins also include hybrids such as epoxy alkyds, epoxy acrylics, epoxy silicone, epoxy silane, epoxy polyurethane, epoxy urethanes, and other modifications are also known. In order to reduce the viscosity of these epoxy resins and 2K blends to a typical viscosity for epoxy coating application of around 2000-4000 cps, dilution with a solvent is often needed. Benzyl alcohol is traditionally used to lower viscosity in solvent epoxy applications. This traditionally requires around 10% benzyl alcohol for viscosity reduction of the epoxy coating. An alternative zero volatile organic compound (VOC)-free epoxy viscosity modifier would be advantageous and preferential over benzyl alcohol.
Benzyl alcohol is also used to improve epoxy reactions by compatibilizing the amine hardener and epoxy. This also helps reduce amine blush. In one aspect of the invention, using certain members of a family of distyryl phenol, tristyryl phenol or cumylphenol ethoxylate-based products as additives to epoxy resins without water reduce the viscosity and modify the pot life and cure time as well as reducing or eliminating amine blush. These additives impart no or very low VOCs to the epoxy coating formulation.
Epoxy resins can alternately be dispersed in water to reduce viscosity without adding VOCs. One technical problem that arises is that epoxy resins are rather hydrophobic, and thus do not readily disperse in water. Therefore, surfactants were developed in the past that would disperse these hydrophobic resins in water. These dispersed resins, however, are not freeze/thaw stable.
Waterborne epoxy resins have been in the marketplace for many years. They are widely accepted as environmentally friendly alternatives to solvent-borne or high solids epoxy systems. They offer distinct advantages over solvent-based epoxy coatings for a number of environmental, safety, and health considerations. They have a lower or zero volatile organic compound (VOC) content which reduces their carbon footprint. Lower VOC formulations reduce air pollution and lead to lower odor, improving customer acceptance. Lower VOCs also contribute to decreased flammability and thus improved safety.
Beyond environmental benefits, waterborne epoxy dispersions also provide further technical advantages to the formulator and applicator. The water-based attribute of these epoxy resin dispersions allows water cleanup. Compared to high solids or 100% solids epoxy formulations, they have significantly lower viscosity contributing to ease of use. These water-dispersed epoxy resins can also be produced at higher molecular weight while maintaining low viscosity, improving flexibility over metal as compared to their high solids or 100% solids counterparts. These high molecular weight epoxy resins also improve set time or walk-on time as compared to solvent-based or high solids epoxies due to their ability to “lacquer dry.” The most important applications for water-based epoxy systems today are coatings on concrete, primers for metal and epoxy cement concrete (ECC).
However, one of the problems with low-VOC waterborne epoxy and hardener dispersions is that the freeze/thaw stability of these dispersions is often poor since common anti-freeze solvents such as propylene glycol are VOCs. Another aspect of the instant invention provides a surfactant system comprising ethoxylates of distyrylphenol, tristyrylphenol or cumylphenol that imparts good freeze/thaw stability to epoxy dispersions. In addition, the stability and pot life of the dispersions are improved, without a concomitant extension of the cure time. This is unusual since pot life and cure time cannot usually be improved simultaneously. Gloss and water resistance of the cured coatings were checked and are good.
In addition, these distyrylphenol, tristyrylphenol or cumylphenol-based ethoxylate surfactants allow the preparation of aqueous epoxy resin dispersions that have good long-term stability at room temperature as well as at elevated temperatures. These dispersions are quite stable, retaining consistent viscosity over extended periods. They also impart good freeze/thaw resistance.

These hydrophobes may be converted into surfactants by methods known in the art such as ethoxylation (nonionic), or by ethoxylation followed by either phosphation or sulfonation to produce anionic end groups which in turn can be neutralized resulting in a counterion cation of sodium, potassium or ammonium.
It is known that surfactants such as those listed in U.S. Pat. No. 6,221,934 may be employed to render the epoxy component emulsifiable. These are nonylphenol ethoxylates, alkylphenol initiated poly(oxyethylene) ethanols, alkylphenol initiated poly(oxypropylene) poly(oxyethylene) ethanols, and block copolymers containing an internal poly(oxypropylene) block and two external poly(oxyethylene) ethanol blocks. In this patent, it is explained that these surfactants do not produce good epoxy dispersions for various end use applications. None of these surfactants are known to produce good freeze-thaw properties in epoxy dispersions. No surfactants are mentioned that use distyryl phenol, tristyryl phenol or cumylphenol hydrophobes.
U.S. Pat. No. 6,271,287B1 cites the use of various surfactants employed in epoxy dispersions. These include long-chain alkyl alkali metal sulfosuccinate such as dioctyl sodium sulfosuccinate, sodium lauryl sulfate, sulfosuccinic acid-4-ester with polyethylene glycol dodecyl ether disodium salt, dialkyl disulfonated diphenyloxide disodium salt. None of these surfactants were shown to produce good freeze-thaw properties in epoxy dispersions. None of the surfactants mentioned use distyryl phenol, tristyryl phenol or cumylphenol hydrophobes.
When epoxy dispersions freeze, ice begins to form within the continuous phase. Thereby the continuous phase expands in volume, or, in other words, the emulsion becomes more concentrated. The pressure on the dispersed droplets increases considerably, and the ice crystals can violate the protective surfactant layer around the emulsion particles. This leads to coalescence of the emulsion droplets, destabilization of the dispersion and separation of the water and epoxy, resulting in a poor coating.
It would therefore be an advantage in the art to discover a waterborne epoxy resin with good freeze-thaw stability.
Finally, another of the problems with state-of-the-art hardeners and waterborne epoxy dispersion mixtures used in coatings, adhesives, damping and other products including epoxy cement concrete coatings, coatings for concrete, primers for metal and other applications is that often the pot life (the usable life of a mixture of an epoxy hardener and an epoxy) is correlated strongly to the cure time (time for the applied material to cure). Thus, if the pot life is very long, so is the cure time. However, a long pot life is desired allowing larger batches to be made, while shorter cure times are desired to allow for earlier use of the finished coated product. It is difficult to simultaneously increase pot life while maintaining or decreasing cure time.
There are few options to increase pot life while maintaining or reducing cure time. One such option is to add acetic acid to enhance pot life; this is undesirable since this adds to the VOC (volatile organic compounds) level. VOCs are being reduced or eliminated in current and future coatings formulations. Acetic acid may also add undesired water sensitivity to the final epoxy coating.
It is known that nonyl phenols are used in hardener applications to modify cure time. Nonyl phenols are used in hardeners such as Ancamine 2368 available from Air Products. In epoxy hardener systems, these traditionally are used to increase compatibility with epoxy materials which decreases cure time but also simultaneously decreases pot life, an undesirable combination. These nonyl phenols are also estrogen mimics and are banned for use in coatings and other uses by many countries.
The reaction adduct of 1,3-bis(aminomethyl)cyclohexane (BAC) with ketones is used but produces inconsistent results. Ketimines are the reaction products of ketones and primary aliphatic amines. In the absence of reactive hydrogens, they do not react with epoxy resins. They can be considered blocked amines or latent hardeners, since they are readily hydrolyzed to regenerate the amines. They have low viscosity, long pot lives and cure rapidly when exposed to atmospheric humidity, and are useful in high solids coatings. Unfortunately, these cannot be used in waterborne coatings due to premature unblocking with water. They also contribute to VOCs and require an added step in the formulation of hardeners.
U.S. Pat. No. 6,271,287B1 cites the use of various surfactants employed in epoxy dispersions. These include long-chain alkyl alkali metal sulfosuccinate such as dioctyl sodium sulfosuccinate, sodium lauryl sulfate, sulfosuccinic acid-4-ester with polyethylene glycol dodecyl ether disodium salt, dialkyl disulfonated diphenyloxide disodium salt. None of these surfactants were shown to produce improved combination of long pot life and short cure times. None of the surfactants mentioned use distyryl phenol, tristyryl phenol or cumylphenol hydrophobes.
It would therefore be an advantage in the art to discover an ingredient in a hardener that would simultaneously increase pot life while maintaining or decreasing cure time.
Distyryl phenol, tristyryl phenol or cumylphenol based additives have not been cited in the patent literature or other published literature for use in producing epoxy resin hardeners. These distyryl phenol, tristyryl phenol or cumylphenol based additives have surprisingly been found to improve both the pot life and cure times of epoxy/hardener systems. Pot life can be increased while cure time is maintained or decreased.